Current cellular system implementations involve the use of a few to the use of many cells to cover a given geographical area. The cells are designed to provide some degree of overlapping coverage. They are also designed to allow reuse of the same channels several cells away (but within the same geographical area).
In practice, cellular system cell-site designs do not cover all the desired coverage areas due to the anomalies of RF propagation. For example, a narrow depression in the terrain such as a ravine or along a road adjacent to a river bed may not have adequate signal coverage due to blockage from nearby terrain. Another example would be in an underground parking garage, or even in large office buildings where larger than normal signal attenuation would result in unacceptable signal levels. Furthermore, cell sites in some cellular systems are not located close enough together, thus resulting in poor coverage areas between the cells.
The addition of new cell sites to remedy such problems is prohibitive in many cases. This is because the numbers of subscribers in these areas are generally of insufficient quantity to justify the cost of a new cell site installation. A low cost alternative solution to this problem is to employ a cellular repeater or booster near the coverage area in question. Such a repeater is intended to retransmit the channels from a nearby (donor) cell into the problem area. The retransmitted channels can then be received by appropriate mobile units in the area. Likewise, transmissions from mobile units in the problem area can be retransmitted by the booster such that they can be heard by the channel receivers at the donor cell site.
Since mobiles are always under control by the cellular system in regard to which channels they are assigned to operate on, a preferred technique for signal boosting is to retransmit on the same channel on which the signal was received. This approach has no impact on the signaling operation of either the cellular system or the mobile, but does require careful control and attention to the installation of the booster to prevent RF feedback oscillation. Separate antennas arranged to maximize isolation are used to provide sufficient margin between the received and retransmitted signals. RF amplifier gain through the retransmission path must be limited to a nominal value of less than the amount of isolation between the two antennas under all operating conditions.
Also, the problem is complicated by the current implementation of the cellular system spectrum. The spectrum is currently split between a "wireline" and a "non-wireline" carrier. Each carrier has available a minimum of 21 control channels to be used for assigning mobiles to voice channels, and for placing and receiving calls to and from the mobiles. The control channel groups of the two carriers are adjacent to each other in the center of the cellular band. The adjacent locations of the control channel groups require special control and coordination between the two carriers to prevent unwanted mobile responses from the other carrier's cell site equipment.
Current cellular booster implementations employ broadband linear amplifiers with filtering to eliminate out-of-band signals. These approaches generally provide a degree of signal enhancement in the area of the booster for a mile or so, which generally is all that is desired in the majority of the cases. However, several problems have been experienced with this solution.
Multiple signals through the broadband linear amplifier create spurious intermodulation products. These products may cause interference with other mobiles and/or stations in the cellular system, interference with competing or adjacent cellular systems, and interference with non-cellular services adjacent to but outside of the cellular bands. Imperfections in even highly-linear amplifiers will cause generation of these unwanted spurious products.
In addition, the competing system's control channel set is immediately adjacent to the control channel set of the target cellular system being repeated. This creates a difficult filtering requirement to prevent the wrong control channels from being amplified. Broadband boosters typically repeat both sets of control channels. This could result in lost mobile calls for the competing system if the booster amplifier did not cover the entire voice band of the competing system.
Finally, an interference region is created on those control channels where signals from the primary source (cell site or mobile) are at or near the same signal level as the boosted or enhanced signal. Signaling completion in these regions is difficult, with many lost calls being a result. Boosted voice signals in these regions are not nearly as affected, since the human ear will integrate out the rapid signal level variations caused by the nearly equal signal levels.
These problems result in limiting the applications and hence the number of areas where such boosters can be installed.
Current known systems employ broadband linear amplifiers for repeating the desired cellular band on an F1-F1 basis (i.e., same frequency out as the same frequency in). Separate antennas usually with highly directive patterns are employed to both achieve isolation between the transmit and the receive antennas, and to minimize the radiation of interfering signals to locations other than the area intended.
To overcome the filtering problem involved in preventing the repeating of the adjacent or competing cellular system's control channels, a combination of a narrow band channel amplifier set to the desired control channel and a broadband amplifier with a reasonably sharp filter for the voice channel set may be used. In this manner, the band pass response of the voice channel broad band filter may be selected such that the competing system's control channels are attenuated sufficiently to prevent improper operation with the competing cellular system.
Also, automatic reduction of the gain of the broadband amplifier via the use of analog AGC circuits has been used to prevent nonlinear operation (and the subsequent unwanted generation of excessive intermodulation products). The disadvantage of this approach, however, is that the weaker signals being repeated may be suppressed to unacceptable levels when nearby mobiles are transmitting. The use of automatic power control by the cellular system may help this situation somewhat; but there will be situations where a nearby mobile may be operating on a cell other than the target "donor" cell, and thus would not be under the control of the donor cell.
The use of individual channel filters to overcome the intermodulation problem has been viewed as uneconomical, since such an approach would require a channel set for every channel that may be installed in the donor cell. Since the area being covered by the booster will generally have a much lower subscriber "population" than that covered by the donor cell, there would be no need to repeat all the donor channels. In fact, there are many areas where a system operator may want to provide coverage but the expected subscriber population is such that only a couple of channels may be needed.
Implementation of the cellular system to determine which channel(s) the cellular booster should repeat would normally require system control of such a complexity that it would be on the order of a standard cell site implementation. Since there could well be many more boosters than cell sites in a given cellular system (i.e., to provide coverage into individual buildings), the designers of the cellular system equipment would understandably be disinclined to dedicate system processing facilities to cover these booster stations. Also, the design of such a cellular booster would be highly dependent on the type of system employed.
Hence, there continues to be a need for cost-effective boosters usable in cellular mobile systems. Such boosters preferably will repeat a limited number of channels without generating spurious signals.